In wireless telecommunication systems, the design of data transmission is a challenging task. The data transmission may be impaired by several factors, such as fast and slow fading, multipath propagation, interference from other systems and interference from other users within the same system.
In general, to transmit signals of a multitude of users simultaneously, several channels are needed. In systems offering large bandwidth to users several channels may be offered to a single user. The channels may be realized with frequencies (frequency division multiple access), time slots (time division multiple access), codes (code division multiple access) and different combinations of the above. One example of an access method studied lately is orthogonal frequency-division multiple access, OFDMA.
Several receiver algorithms have been designed to wireless telecommunication systems. The receiver must be able to capture and demodulate the transmitted signal as efficiently as possible. Maximum Ratio Combining (MRC), Maximum Likelihood receiver and Maximum A Posteriori Receiver (MAP) are examples of known receiver algorithms.
For environments where interference is present, interference suppression methods have been developed. An example of such a method is Interference Rejection Combiner, which is an efficient method in applications where multiple antennas are utilized in receivers.
In all algorithms and methods mentioned above, the calculation of a signal covariance matrix is required. Covariance matrix can be formed or estimated using various methods. In some cases covariance matrix is calculated using input samples of the received signal. In other cases, a noise covariance matrix is calculated. The first method does not produce so reliable results as the calculation of noise covariance matrix. However, present methods of determining a noise covariance matrix rely on channel estimates. Thus, channel transfer functions must be calculated prior to the determination of the noise covariance matrix. This causes additional delay as the calculation of channel transfer functions is a time consuming process.